A resident walks down a hallway inside the Nightingale migrant labor camp in Rantoul, Ill., in July 2014. Companies in Illinois received approval for only five H-2A workers from October to December 2015.

Hundreds of migrant workers come to the United States from Mexico and other countries with special H-2A farm visas, but they make up only a fraction of the total number of migrant workers.

The H-2A program federally requires that workers live in free housing inspected by state and local authorities. That housing can include licensed labor “camps,” another common term for migrant housing, but does not have to.

At least six out of 10 farmworkers are undocumented, meaning they are in the country illegally, according to a study by the Southern Poverty Law Center.

“What we have is a group of disenfranchised people in this country, many of whom are third and fourth generation agricultural workers who live in some of the most impoverished counties in the United States where there isn’t enough employment to sustain them on a 12-month, year-round basis,” said Bobbi Ryder, head of the National Center for Farmworker Health, a Texas nonprofit organization. “It is a relatively invisible population.”